U.S. patent number 6,863,738 [Application Number 09/771,186] was granted by the patent office on 2005-03-08 for method for removing oxides and coatings from a substrate.
This patent grant is currently assigned to General Electric Company. Invention is credited to Lawrence Bernard Kool, James Anthony Ruud.
United States Patent |
6,863,738 |
Kool , et al. |
March 8, 2005 |
Method for removing oxides and coatings from a substrate
Abstract
A method for selectively removing oxide material from the
surface of a substrate or coating disposed on the substrate is
disclosed. The method includes the step of contacting the oxide
material with an aqueous treatment composition having the formula
H.sub.x AF.sub.6, wherein A can be Si, Ge, Ti, Zr, Al, and Ga; and
x is 1-6. The composition can sometimes include an additional acid,
such as phosphoric acid, nitric acid, sulfuric acid, hydrochloric
acid, hydrofluoric acid, and mixtures thereof. A method for
replacing a worn or damaged protective coating applied over a
substrate, utilizing the treatment composition, is also
described.
Inventors: |
Kool; Lawrence Bernard (Clifton
Park, NY), Ruud; James Anthony (Delmar, NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
25090982 |
Appl.
No.: |
09/771,186 |
Filed: |
January 29, 2001 |
Current U.S.
Class: |
134/3;
134/41 |
Current CPC
Class: |
C23C
4/02 (20130101); C23G 1/10 (20130101); C23C
28/00 (20130101); C23C 10/02 (20130101) |
Current International
Class: |
C23G
1/10 (20060101); C23C 10/00 (20060101); C23G
1/02 (20060101); C23C 10/02 (20060101); C23C
4/02 (20060101); C23C 28/00 (20060101); B08B
007/00 () |
Field of
Search: |
;216/96,97,100,101,102,103,104,108,109 ;134/1,2,3,41 ;252/79.2,79.3
;510/255,269,405 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
1162286 |
|
Dec 2001 |
|
EP |
|
2115013 |
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Sep 1983 |
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GB |
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56096083 |
|
Aug 1981 |
|
JP |
|
56166386 |
|
Dec 1981 |
|
JP |
|
WO9113186 |
|
Sep 1991 |
|
WO |
|
WO9303198 |
|
Feb 1993 |
|
WO |
|
Primary Examiner: Markoff; Alexander
Attorney, Agent or Firm: Cantor Colburn LLP
Claims
What is claimed:
1. A method for removing at least one of: (1) an oxidized product
of a substrate from a surface of the substrate, wherein the
substrate is a turbine component formed of an alloy selected from
the group consisting of a nickel based alloy, a cobalt based alloy,
and an iron based alloy, or (2) an oxidized product of a metallic
coating disposed on the substrate from a surface of the metallic
coating, wherein the substrate is the turbine component formed of
the alloy, the method comprising the step of contacting the
oxidized product of the substrate or the oxidized product of the
metallic coating with an aqueous composition to remove a
predetermined amount of the oxidized product of the substrate or a
predetermined amount of the oxidized product of the metallic
coating, wherein the aqueous composition consists essentially of an
acid having the formula H.sub.x AF.sub.6 and water, wherein A is
selected from the group consisting of Si, Ge, Ti, and Ga; and x is
1-6.
2. The method of claim 1, wherein x is 1-3.
3. The method of claim 1, wherein the acid is present at a level in
the range of about 0.05 M to about 5 M.
4. The method of claim 3, wherein the acid is present at a level in
the range of about 0.2 M to about 3.5 M.
5. The method of claim 1, wherein the aqueous composition is
H.sub.2 SiF.sub.6.
6. A method for removing at least one of an oxidized product of a
substrate from a surface of the substrate, wherein the substrate is
a turbine component formed of an alloy selected from the group
consisting of a nickel based alloy, a cobalt based alloy, and an
iron based alloy an alloy comprising nickel, chromium, aluminum, or
at least one of the foregoing metals, or a polymer, or an oxidized
product of a metallic coating disposed on the substrate from a
surface of the metallic coating, wherein the substrate is the
turbine component formed of the alloy, the method comprising the
step of contacting the oxidized product of the substrate or the
oxidized product of the metallic coating with an aqueous
composition to remove a predetermined amount of the oxidized
product of the substrate or a predetermined amount of the oxidized
product of the metallic coating, wherein the aqueous composition
consists essentially of an acid having the formula H.sub.x
AF.sub.6, at least one additional acid, and water, wherein A is
selected from the group consisting of Si, Ge, Ti, and Ga; and x is
1-6.
7. The method of claim 6, wherein the at least one additional acid
has a pH of less than about 7 in water.
8. The method of claim 7, wherein the at least one additional acid
has a pH of less than about 3.5 in water.
9. The method of claim 6, wherein the at least one additional acid
is a mineral acid.
10. The method of claim 6, wherein the at least one additional acid
is selected from the group consisting of phosphoric acid, nitric
acid, sulfuric acid, hydrochloric acid, hydrofluoric acid,
hydrobromic acid, hydriodic acid, acetic acid, perchloric acid,
phosphorous acid, phosphinic acid, alkyl sulfonic acids, and
mixtures of any of the foregoing.
11. The method of claim 6, wherein the at least one additional acid
is phosphoric acid.
12. The method of claim 6, wherein the at least one additional acid
is present at a level less than about 80 mole %, based on the total
moles of acid present in the aqueous composition.
13. The method of claim 12, wherein the at least one additional
acid is present at a level of about 20 mole % to about 70 mole
%.
14. The method of claim 1, wherein the oxide material is treated in
a bath of the aqueous composition.
15. The method of claim 14, wherein the bath is maintained at a
temperature in the range of about room temperature to about
100.degree. C., during treatment.
16. The method of claim 15, wherein the temperature is in the range
of about 45.degree. C. to about 90.degree. C.
17. The method of claim 15, wherein the treatment time is in the
range of about 10 minutes to about 72 hours.
18. The method of claim 17, wherein the treatment time is in the
range of about 60 minutes to about 20 hours.
19. A method for removing at least one of: (1) an oxidized product
of a substrate from a surface of the substrate, wherein the
substrate is a turbine component formed of an alloy selected from
the group consisting of a nickel based alloy, a cobalt based alloy,
and an iron based alloy, or (2) an oxidized product of a metallic
coating disposed on the substrate from a surface of the metallic
coating, wherein the substrate is the turbine component formed of
the alloy, the method comprising the step of exposing the oxidized
product of the substrate or the oxidized product of the metallic
coating to an aqueous composition to remove a predetermined amount
of the oxidized product of the substrate or a predetermined amount
of the oxidized product of the metallic coating, wherein the
aqueous composition consists essentially of an acid having the
formula H.sub.x AF.sub.6 and water, wherein A is selected from the
group consisting of Si, Ge, Ti, and Ga; and x is 1-6, and wherein
the precursors to said acid comprise any compound or group of
compounds which can be combined to form the acid or its dianion
AF.sub.6.sup.-2.
20. A method for removing an oxide material from a diffusion- or
overlay coating on the surface of a turbine engine component,
comprising the step of contacting the oxide material with an
aqueous composition to selectively remove the oxide material from
the diffusion or the overlay coating, wherein the aqueous
composition comprises H.sub.2 SiF.sub.6, wherein the diffusion
coating comprises an aluminide alloy, and wherein the overlay
coating comprises a composition having a formula of MCrAl(X),
wherein M is an element selected from the group consisting of Ni,
Co, Fe, and combinations thereof, and wherein X is an element
selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C,
and combinations thereof.
21. The method of claim 20, wherein the aqueous composition further
comprises an additional acid selected from the group consisting of
phosphoric acid, nitric acid, sulfuric acid, hydrochloric acid,
hydrofluoric acid, and mixtures thereof, wherein the additional
acid is present at a level less than about 80 mole %, based on the
total moles of acid present in the aqueous composition.
22. The method of claim 20, wherein the oxide material is also
initially present in at least one cavity within the turbine engine
component, and is removed therefrom during treatment with the
aqueous composition.
23. A method for replacing a protective coating applied over a
substrate, comprising the following steps: (i) removing an oxide
material from a surface of the protective coating disposed on the
substrate by contacting the oxide material with an aqueous
composition which comprises an acid having the formula H.sub.x
AF.sub.6, or precursors to said acid, wherein A is selected from
the group consisting of Si, Ge, Ti, and Ga; and x is 1-6; (ii)
removing the protective coating disposed on the substrate by
contacting the protective coating with the aqueous composition; and
(iii) applying a new protective coating to the substrate.
24. The method of claim 23, wherein steps (i) and (ii) are carried
out simultaneously, using the same aqueous composition.
25. The method of claim 24, wherein the aqueous composition further
comprises at least one additional acid or precursor thereof.
26. The method of claim 25, wherein the additional acid is selected
from the group consisting of phosphoric acid, nitric acid, sulfuric
acid, hydrochloric acid, hydrofluoric acid, hydrobromic acid,
hydriodic acid, acetic acid, perchloric acid, phosphorous acid,
phosphinic acid, alkyl sulfonic acids, and mixtures of any of the
foregoing.
27. The method of claim 23, wherein the coating removed in step
(ii) and the coating applied in step (iii) are each selected from
the group consisting of diffusion coatings and overlay
coatings.
28. The method of claim 23, wherein the new coating of step (iii)
is applied by a technique selected from the group consisting of
vacuum plasma spray (VPS); air plasma spray (APS); high velocity
oxy-fuel (HVOF); sputtering; physical vapor deposition (PVD);
electron beam physical vapor deposition (EB-PVD); and
diffusion-aluminiding.
Description
BACKGROUND OF THE INVENTION
In a general sense, this invention relates to methods for removing
material applied to or formed over a metal substrate. More
specifically, it relates to methods for removing an oxide material
which is disposed on a substrate, or on a coating applied over the
substrate.
Metal alloys are often used in industrial environments which
include extreme operating conditions. As an example, gas turbine
engines are often subjected to repeated thermal cycling during
operation. The standard operating temperature of turbine engines
continues to be increased, to achieve improved fuel efficiency. The
turbine engine components (and other industrial parts) are often
formed of superalloys, which can withstand a variety of extreme
operating conditions. However, they often must be covered with
coatings which protect them from environmental degradation, e.g.,
the adverse effects of corrosion and oxidation. Current coatings
used on components in gas turbine hot sections, such as blades,
nozzles, combustors, and transition pieces, generally belong to one
of two classes: diffusion coatings or overlay coatings.
State-of-the-art diffusion coatings are generally formed of
aluminide-type alloys, such as nickel-aluminide; a noble
metal-aluminide such as platinum aluminide; or
nickel-platinum-aluminide. Overlay coatings typically have the
composition MCrAl(X), where M is an element selected from the group
consisting of Ni, Co, Fe, and combinations thereof, and X is an
element selected from the group consisting of Y, Ta, Si, Hf, Ti,
Zr, B, C, and combinations thereof. Diffusion coatings are formed
by depositing constituent components of the coating, and reacting
those components with elements from the underlying substrate, to
form the coating by high temperature diffusion. In contrast,
overlay coatings are generally deposited intact, without reaction
with the underlying substrate.
During service, diffusion and overlay coatings on a component are
often exposed to oxidative conditions. For example, coatings on
turbine airfoils are typically subjected to oxidation in the hot
gas path during normal operation. Under such conditions, which
often include temperatures in the range of about 1400-2100.degree.
F. (about 760-1149.degree. C.), various oxidative products (mainly
thermally-grown oxide or "TGO") are formed on the coatings. For
example, aluminum oxides (especially .alpha.-aluminum oxides) often
form on platinum-aluminide coatings. Aluminum oxides, chromium
oxides, and various spinels often form on the MCrAl(X)-type
coatings.
When turbine engine components are overhauled, the protective
coatings are often removed to allow inspection and repair of the
underlying substrate. Various stripping compositions have been used
to remove the coatings. Usually, the oxide materials must be
removed before the coatings can be treated with the stripping
composition.
In past practice, oxide removal in this situation has been carried
out as a separate step, prior to removal of the underlying coating.
Various techniques have been used for oxide removal. For example,
the oxide materials have often been removed from external sections
of the turbine component by grit blasting.
As an alternative, the turbine component has sometimes been treated
in an oxide-removal solution, i.e., one separate from the stripping
composition used to subsequently remove the protective coating.
These solutions have usually been based on strong mineral acids or
strong caustics. Examples of the mineral acids are hydrochloric
acid, sulfuric acid, and nitric acid. The caustic solutions usually
include sodium hydroxide, potassium hydroxide, or various molten
salts. Repeated treatments sometimes have to be used to remove the
oxide. After removal of the oxide is completed, the substrate is
then typically immersed in another solution--one that is suitable
for removing the coating material itself.
These oxide removal techniques are sometimes effective, but there
are often drawbacks to their use. For example, grit blasting is a
labor-intensive process that is usually carried out on a
piece-by-piece basis. Special care must sometimes be taken, to
prevent grit-blasting damage to the substrate or any protective
coating not being removed during the turbine component overhaul.
Moreover, grit blasting cannot generally be used to remove oxide
material from internal passage holes or cavities in the
component.
Use of the oxide removal solution is advantageous in some
situations, but also has drawbacks. For example, the use of two
separate treatment solutions (one for removing the oxide and the
other for removing the coating material) is not always desirable. A
considerable amount of processing time is often involved, which can
lower productivity in an industrial setting. Moreover, conventional
treatment solutions which employ large quantities of strong mineral
acids may emit an excessive amount of hazardous, acidic fumes. Due
to environmental, health and safety concerns, such fumes must be
scrubbed from ventilation exhaust systems.
Thus, new processes for removing oxide materials from coatings
and/or from metal substrates would be welcome in the art. The
processes should not result in the formation of an unacceptable
amount of hazardous fumes. It would also be helpful if the
processes were capable of removing a substantial amount of oxide
material that might be located in indentations, hollow regions, or
holes in the substrate, e.g., passage holes in a turbine engine
substrate. Moreover, the processes should preferably be capable of
being combined with other processing steps, such as a coating
removal step.
SUMMARY OF THE INVENTION
A primary embodiment of this invention is directed to a method for
removing an oxide material from the surface of a substrate or a
coating disposed on the substrate. The method includes the step of
contacting the oxide material with an aqueous composition which
comprises an acid having the formula H.sub.x AF.sub.6, or
precursors to said acid, wherein A is selected from the group
consisting of Si, Ge, Ti, Zr, Al, and Ga; and x is 1-6. The acid is
usually present at a level in the range of about 0.05 M to about 5
M, and is often either H.sub.2 SiF.sub.6 or H.sub.2 ZrF.sub.6, or
mixtures thereof. Treatment is usually carried out by immersion in
a bath of the aqueous composition. In some embodiments, the bath
includes an additional acid, such as phosphoric acid, nitric acid,
sulfuric acid, hydrochloric acid, hydrofluoric acid, and mixtures
thereof.
Another aspect of the present invention is directed to a method for
removing a coating from a substrate (e.g., a diffusion- or overlay
coating), along with the oxide material which generally is disposed
on top of the coating. The present inventors have discovered that
the coating and the oxide material can be removed in a single step,
by exposure to the same treatment composition, which is mentioned
above and further described below. Moreover, the underlying
substrate is not adversely affected by the treatment. Furthermore,
in contrast to prior art techniques like grit blasting, the present
method can be used to effectively remove oxide material from the
internal sections of the substrate.
A method for replacing a worn or damaged protective coating applied
over a substrate also constitutes part of the present invention.
The method includes the step of cleaning the substrate by removing
oxide material and coating material, using the treatment
composition described below. A new protective coating is then
applied to the substrate by various techniques.
Further details regarding the various features of this invention
are found in the remainder of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of an external portion of a
turbine bucket, including a coating and oxidized material, after
treatment according to this invention.
FIG. 2 is a cross-sectional view of an internal portion of a coated
and oxidized turbine bucket, after treatment according to this
invention.
FIG. 3 is a cross-sectional view of another section of an internal
portion of a coated and oxidized turbine bucket, after treatment
according to this invention.
FIG. 4 is a cross-sectional view of a sample coupon which includes
a coating and oxide material.
FIG. 5 is a cross-sectional view of the coupon of FIG. 4, after
partial treatment according to the present invention.
FIG. 6 is a cross-sectional view of the coupon of FIG. 5, after
further treatment according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
As alluded to earlier, the actual configuration of a substrate may
vary widely. As a general illustration, the substrate may be in the
form of a houseware item (e.g., cookware), or a printed circuit
board substrate. Very often, the substrate is a turbine engine
component, as further exemplified below.
As used herein, the term "oxide material" is generally meant to
include the oxidized product or products of any metallic coating
applied on a substrate, or the oxidized products of the substrate
itself. In most cases (but not always), these products are formed
on the coating after it has been exposed to the elevated
temperatures mentioned above, i.e., about 1400.degree. F.
(760.degree. C.) to about 2100.degree. F. (1149.degree. C.).
Examples of the metallic coatings are diffusion coatings and
overlay coatings, described above, and in patent application Ser.
No. 09/591,531 of L. Kool et al, filed on Jun. 9, 2000, and
incorporated herein by reference. (It should also be noted that the
term "oxide" is meant to include the various phases of the oxide,
e.g., alpha-alumina and alpha-chromia.)
The term "oxide material" also includes the oxidized product or
products of the substrate material itself, in those locations where
no coating is present. As an example, the surface of a nickel-based
substrate exposed to elevated temperatures for extended periods of
time will at least partially be transformed into various metal
oxides (depending on the substrate's specific composition), such as
aluminum oxide, chromium oxide, nickel oxide, cobalt oxide, and
yttrium oxide. Various spinels may also form, such as
Ni(Cr,Al).sub.2 O.sub.4 spinels and Co(Cr,Al).sub.2 O.sub.4
spinels. In the case of a platinum-nickel-aluminide coating, the
oxidation product is primarily aluminum oxide (e.g., alpha-alumina
and/or gamma alumina), and possibly nickel oxide.
The oxide material may be located in a variety of locations on a
component, and is usually (but not always) formed over a protective
coating, as described previously. In the case of a turbine engine,
the oxide material is often formed on coatings which are applied on
combustor liners, combustor domes, shrouds, or airfoils, including
buckets or blades, and nozzles or vanes. The oxide material can be
found on the flat areas of substrates, as well as on curved or
irregular surfaces.
The oxide material is also formed on the surfaces of cavities in
the substrates, e.g., indentations, hollow regions, or holes. For
example, the cavities can be in the form of radial cooling holes or
serpentine passageways, which can have an overall length of up to
about 30 inches (76.2 cm). It is often very difficult to remove the
oxide material from the surface of these cavities by conventional,
line-of-sight processes, such as grit blasting, plasma etching, or
laser ablation.
The thickness of the oxide material will depend on a variety of
factors. These include the length of service time for the
component; its thermal history; and the particular composition of
the underlying coating (or substrate). Usually a layer of oxide
material has a thickness in the range of about 0.5 micron to about
20 microns, and most often, in the range of about 1 micron to about
10 microns.
A variety of substrates may include the oxide material being
removed according to this invention. Usually, the substrate is a
metallic material or a polymeric (e.g., plastic) material. As used
herein, "metallic" refers to substrates which are primarily formed
of metal or metal alloys, but which may also include some
non-metallic components. Non-limiting examples of metallic
materials are those which comprise at least one element selected
from the group consisting of iron, cobalt, nickel, aluminum,
chromium, titanium, and mixtures which include any of the foregoing
(e.g., stainless steel).
Very often, the metallic material is a superalloy, as described in
the previously-referenced patent application, Ser. No. 09/591,531.
The superalloy is typically nickel-, cobalt-, or iron-based,
although nickel- and cobalt-based alloys are favored for
high-performance applications. The base element, typically nickel
or cobalt, is the single greatest element in the superalloy by
weight. Illustrative nickel-base superalloys include at least about
40 wt % Ni, and at least one component from the group consisting of
cobalt, chromium, aluminum, tungsten, molybdenum, titanium, and
iron. Examples of nickel-base superalloys are designated by the
trade names Inconel.RTM., Nimonic.RTM., and Rene.RTM., and include
directionally solidified and single crystal superalloys.
Illustrative cobalt-base superalloys include at least about 30 wt %
Co, and at least one component from the group consisting of nickel,
chromium, aluminum, tungsten, molybdenum, titanium, and iron.
Examples of cobalt-base superalloys are designated by the trade
names Haynes.RTM., Nozzaloy.RTM., Stellite.RTM. and
Ultimet.RTM..
Polymeric substrates which can be treated by this invention are
formed from materials which are substantially acid-resistant. In
other words, such materials are not adversely affected by the
action of the acid (or acids), to the degree which would make the
substrate unsuitable for its intended end use. (Usually, such
materials are highly resistant to hydrolysis). Non-limiting
examples of such materials are polyolefins (e.g., polyethylene or
polypropylene), polytetrafluoroethylenes, epoxy resins,
polystyrenes, polyphenylene ethers; mixtures comprising one of the
foregoing; and copolymers comprising one of the foregoing. Those
skilled in the polymer arts understand that the properties of an
individual polymer may be modified by various methods, e.g.,
blending or the addition of additives. (Oxide layers are not
typically formed on polymeric materials, in the way that they are
formed on metals. Thus, in the case of a polymeric substrate, the
claimed process would usually be undertaken to remove oxide
material from metallic coatings (e.g., aluminide) which have been
deposited on top of the polymeric substrate.)
As mentioned above, the aqueous composition for some embodiments of
this invention includes an acid having the formula H.sub.x
AF.sub.6. In this formula, A is selected from the group consisting
of Si, Ge, Ti, Zr, Al, and Ga. The subscript "x" is a quantity from
1 to 6, and more typically, from 1 to 3. Materials of this type are
available commercially, or can be prepared without undue effort.
The preferred acids are H.sub.2 SiF.sub.6, H.sub.2 ZrF.sub.6, or
mixtures thereof. In some embodiments, H.sub.2 SiF.sub.6 is
especially preferred. The last-mentioned material is referred to by
several names, such as "hydrofluosilicic acid", "fluorosilicic
acid", and "hexafluorosilicic acid".
Precursors to the H.sub.x AF.sub.6 acid may also be used. As used
herein, a "precursor" refers to any compound or group of compounds
which can be combined to form the acid or its dianion
AF.sub.6.sup.-2, or which can be transformed into the acid or its
dianion under reactive conditions, e.g. the action of heat,
agitation, catalysts, and the like. Thus, the acid can be formed in
situ in a reaction vessel, for example.
As one illustration, the precursor may be a metal salt, inorganic
salt, or an organic salt in which the dianion is ionically bound.
Non-limiting examples include salts of Ag, Na, Ni, K, and
NH.sub.4.sup.+, as well as organic salts, such as a quaternary
ammonium salt. Dissociation of the salts in an aqueous solution
yields the acid. In the case of H.sub.2 SiF.sub.6, a convenient
salt which can be employed is Na.sub.2 SiF.sub.6.
Those skilled in the art are familiar with the use of compounds
which cause the formation of H.sub.x AF.sub.6 within an aqueous
composition. For example, H.sub.2 SiF.sub.6 can be formed in situ
by the reaction of a silicon-containing compound with a
fluorine-containing compound. An exemplary silicon-containing
compound is SiO.sub.2, while an exemplary fluorine-containing
compound is hydrofluoric acid (i.e., aqueous hydrogen fluoride,
HF).
When used as a single acid, the H.sub.x AF.sub.6 acid appears to be
somewhat effective for removing the oxide materials described
above. The preferred level of acid employed will depend on various
factors, such as the type and amount of oxide material being
removed; the location of the oxide material on (or within) a
substrate; the type of coating material and substrate; the thermal
history of the coating material and substrate; the technique by
which the oxide material is being exposed to the treatment
composition (as described below); the time and temperature used for
treatment; and the stability of the acid in solution.
In general, the H.sub.x AF.sub.6 acid is present in a treatment
composition at a level in the range of about 0.05 M to about 5 M,
where M represents molarity. (Molarity can be readily translated
into weight or volume percentages, for ease in preparing the
solutions). Usually, the level is in the range of about 0.2 M to
about 3.5 M. In the case of H.sub.2 SiF.sub.6, a preferred
concentration range is often in the range of about 0.2 M to about
2.2 M. Adjustment of the amount of H.sub.x AF.sub.6 acid, and of
other components described below, can readily be made by observing
the effect of particular compositions on oxide removal from the
underlying coating or substrate.
In some preferred embodiments, the aqueous composition may contain
at least one additional acid, i.e., in addition to the "primary"
acid, H.sub.x AF.sub.6. It appears that the use of the additional
acid (the "secondary" acid or acids) often enhances the removal of
oxide material from less accessible areas of the substrate. A
variety of different acids can be used, and they are usually
characterized by a pH less than about 7 in pure water. In preferred
embodiments, the additional acid has a pH of less than about 3.5 in
pure water. In some especially preferred embodiments, the
additional acid has a pH which is less than the pH (in pure water)
of the primary acid, i.e., the H.sub.x AF.sub.6 material. Thus, in
the case of H.sub.2 SiF.sub.6, the additional acid is preferably
one having a pH less than about 1.3.
Various types of acids may be used, e.g., a mineral acid or an
organic acid. Non-limiting examples include phosphoric acid, nitric
acid, sulfuric acid, hydrochloric acid, hydrofluoric acid,
hydrobromic acid, hydriodic acid, acetic acid, perchloric acid,
phosphorous acid, phosphinic acid, alkyl sulfonic acids (e.g.,
methanesulfonic acid), and mixtures of any of the foregoing.
(Sometimes, the acids are advantageously supplied and used in
aqueous form, e.g., 35-38% hydrochloric acid in water). Those
skilled in the art can select the most appropriate additional acid,
based on observed effectiveness and other factors, such as
availability, compatibility with the primary acid, cost, and
environmental considerations. Moreover, a precursor of the acid may
be used (e.g., a salt), as described above in reference to the
primary acid. In some preferred embodiments of this invention, the
additional acid is selected from the group consisting of phosphoric
acid, nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric
acid, and mixtures thereof. In some especially preferred
embodiments (e.g., when the primary acid is H.sub.2 SiF.sub.6), the
additional acid is phosphoric acid.
The amount of additional acid employed will depend on the identity
of the primary acid, and on many of the factors set forth above.
When used, the additional acid is preferably present at a level
less than about 80 mole %, based on the total moles of acid present
in the treatment composition. In some preferred embodiments, the
additional acid is present at a level within the range of about 20
mole % to about 70 mole %. Furthermore, some especially preferred
embodiments contemplate a range of about 20 mole % to about 35 mole
%. As alluded to earlier, longer treatment times and/or higher
treatment temperatures may compensate for lower levels of the acid,
and vice versa. Experiments can be readily carried out to determine
the most appropriate level for the additional acid. (The process of
the present invention is generally free of the problems associated
with prior art processes which required relatively large amounts of
strong acids, as described previously).
The aqueous composition of the present invention may include
various other additives which serve a variety of functions.
Non-limiting examples of these additives are inhibitors,
dispersants, surfactants, chelating agents, wetting agents,
deflocculants, stabilizers, anti-settling agents, reducing agents,
and anti-foam agents. Those of ordinary skill in the art are
familiar with specific types of such additives, and with effective
levels for their use. An example of an inhibitor for the
composition is a relatively weak acid like acetic acid, mentioned
above. Such a material tends to lower the activity of the primary
acid in the composition. This is desirable in some instances, e.g.,
to decrease the potential for pitting of the substrate surface if
it is contacted with the treatment composition.
Various techniques can be used to treat the substrate with the
aqueous composition. For example, the substrate can be continuously
sprayed with the composition, using various types of spray guns. A
single spray gun could be employed. Alternatively, a line of guns
could be used, and the substrate could pass alongside or through
the line of guns (or multiple lines of guns). In another
alternative embodiment, the oxide-removal composition could be
poured over the substrate (and continuously recirculated).
In preferred embodiments, the substrate is immersed in a bath of
the aqueous composition. Immersion in this manner (in any type of
vessel) often permits the greatest degree of contact between the
aqueous composition and the oxide material being removed. Immersion
time and bath temperature will depend on many of the factors
described above, such as the type of oxide being removed, and the
acid (or acids) being used in the bath. Usually, the bath is
maintained at a temperature in the range of about room temperature
to about 100.degree. C., while the substrate is immersed therein.
In preferred embodiments, the temperature is maintained in the
range of about 45.degree. C. to about 90.degree. C. The immersion
time may vary considerably, but is usually in the range of about 10
minutes to about 72 hours, and preferably, from about 1 hour to
about 20 hours. Longer immersion times may compensate for lower
bath temperatures. After removal from the bath (or after contact of
the coating by any technique mentioned above), the substrate is
typically rinsed in water, which also may contain other
conventional additives, such as a wetting agent.
An important advantage of the present invention is that an oxide
material can be removed from a coating in the same step that the
coating is being removed from an underlying substrate. For example,
exposure of the substrate to a treatment solution as described
above removes substantially all of the oxide material, and then
removes substantially all of the coating, e.g., a diffusion or
overlay coating. Details regarding the removal of these types of
coatings from metal or polymeric substrates are set forth in the
above-referenced patent application, Ser. No. 09/591,531.
Thus, another embodiment of this invention is directed to a method
for cleaning a substrate, i.e., removing substantially all oxide
material and coating material from its surface. The method
comprises exposing the substrate to a treatment composition (as
described previously), under conditions sufficient to remove the
oxide material and any coating material. In general, the oxide
material is stripped first, followed by removal of the underlying
coating. However, there may be some overlap, i.e., portions of the
oxide material and coating material may be removed from the
substrate simultaneously.
The period of time required to remove both the oxide material and
the coating from a substrate will vary substantially, depending on
the factors set forth above, e.g., the composition and thickness of
the oxide material and coating material; as well as the temperature
of the treatment composition. In general, the time period will be
within about 10% to about 50% greater than the time period needed
for a single treatment, if the treatments were carried out in two
separate steps, e.g., in two separate stripping baths. For example,
in the case of an oxidized aluminide or platinum-aluminide coating
having a total thickness in the range of about 5 microns to about
10 microns, the overall treatment time will usually be in the range
of about 10 minutes to about 20 hours. The bath temperature is
usually maintained within the range described previously.
Another aspect of the present invention is directed to a method for
replacing a worn or damaged protective coating applied over a
substrate. As mentioned earlier, oxides form on metallic coatings
which have been in service, e.g., turbine engine components. These
oxides have to be removed before the underlying coating can be
repaired or replaced. Thus, the method comprises the following
steps:
(i) removing an oxide material from the surface of a coating
disposed on the substrate, by contacting the oxide material with an
aqueous composition which comprises an acid having the formula
H.sub.x AF.sub.6, or precursors to said acid, wherein A is selected
from the group consisting of Si, Ge, Ti, Zr, Al, and Ga; and x is
1-6;
(ii) removing the coating disposed on the substrate, by contacting
the coating with an aqueous composition which comprises an acid
having the formula H.sub.x AF.sub.6, or precursors to said acid,
wherein A is selected from the group consisting of Si, Ge, Ti, Zr,
Al, and Ga; and x is 1-6; and then
(iii) applying a new coating to the substrate.
As described earlier, the same aqueous composition can be used for
steps (i) and (ii). Moreover, techniques for applying the new
coating are well-known in the art. As an example, various thermal
spray techniques can be employed for the deposition of the overlay
coatings. Examples include vacuum plasma spray (VPS), air plasma
spray (APS), and high velocity oxy-fuel (HVOF). Other deposition
techniques could be used as well, such as sputtering and physical
vapor deposition (PVD), e.g., electron beam physical vapor
deposition (EB-PVD).
Various techniques are also well-known for applying diffusion
coatings, e.g., noble metal-aluminide coatings such as
platinum-aluminide or palladium-aluminide. As an example in the
case of platinum-aluminide, platinum can initially be electroplated
onto the substrate, using P-salt or Q-salt electroplating
solutions. In a second step, the platinum layer is
diffusion-treated with aluminum vapor to form the
platinum-aluminide coating. This technique is sometimes referred to
as "diffusion-aluminiding".
The examples which follow are merely illustrative, and should not
be construed to be any sort of limitation on the scope of the
claimed invention.
EXAMPLE 1
The substrate for this example was a gas turbine bucket formed from
a directionally-solidified, nickel-base superalloy. The bucket
included a number of cavities, most of which were in the shape of
serpentine passage holes, forming a cooling circuit. The bucket had
initially been coated by VPS with an MCrAlY-type material, having
an approximate, nominal composition as follows: 29 wt % Cr, 6 wt %
Al, 1 wt % Y, balance Co. The coating was applied by a thermal
spray technique, to a thickness of about 250 microns. The coated
surface was then diffusion-aluminided to a depth of about 50
microns. The cavities were also diffusion-aluminided.
The gas turbine bucket had a service life of about 24,000 hours.
This exposure resulted in oxide formation on the coating, in both
external regions and internal (i.e., within passage holes) regions.
The oxide depth varied to some extent, but was generally in the
range of about 1 micron to about 10 microns.
The bucket was immersed in a solution of 75 volume % fluorosilicic
acid (H.sub.2 SiF.sub.6, at 23 wt % concentration) and 25 volume %
phosphoric acid (86 wt % concentration), and vigorously stirred at
70.degree. C. After 13 hours, the oxide and coating material had
been stripped from both external and internal surfaces.
FIG. 1 is a photomicrograph of an external cross-section of a
portion of the turbine bucket after treatment according to this
invention. (The grain structure of the metallic cross-section has
been highlighted, using a grain etch.) The figure depicts the
"suction side" of the bucket's 10% span. Section A is the
substrate, while section B is a depletion zone, i.e., the zone
where the aluminum has actually been depleted from the base
metal.
FIG. 2 is a photomicrograph of an internal cross-section of a
portion of the turbine bucket after treatment is complete. The
figure depicts a section of a passage hole, with section A showing
the substrate. The areas marked as elements "C" in this figure are
the eutectic phase . The eutectic phase is often present in this
type of substrate metal, and is very susceptible to attack by
conventional stripping techniques, e.g., using strong mineral
acids.
FIG. 3 is a photomicrograph of an internal cross-section of another
portion of the turbine bucket after the completion of treatment.
(The orientation is vertical in this figure, with section A again
depicting the substrate). This figure also demonstrates
substantially complete removal of the oxide and coating. Moreover,
the treatment did not result in detrimental attack on the eutectic
phase C.
EXAMPLE 2
In this experiment, the substrate was a sample coupon of a
directionally-solidified, nickel-base superalloy material similar
to the bucket composition described in Example 1. The coupon was
coated by HVOF with the MCrAlY material described in the previous
example (coating depth of about 200 to 300 microns), and then
over-aluminided in a similar manner.
The coupon was heated at 2050.degree. F. (1121.degree. C.) in air
for 47 hours, in order to simulate the oxidation that would occur
under normal operating conditions. The coupon was then immersed in
a treatment bath identical to that described in Example 1. The
coupon was removed from the bath for sampling at periodic
intervals.
In FIGS. 4-6, element D delineates the substrate. In FIG. 4,
section E is a diffusion zone, while section F is the oxide
material. The thin line of material designated as element G is the
over-aluminided coating material, although the distinction between
"oxide" and "coating" is not especially clear after severe
oxidation has occurred.
FIGS. 5 and 6 demonstrate progressive dissolution of the oxide
material at treatment times of 6 hours and 8 hours, respectively.
FIG. 6 shows substantially complete removal of the oxide material,
with only residual material remaining. This residual material is
very porous, and easily-removed at this stage.
Some of the preferred embodiments have been set forth in this
disclosure for the purpose of illustration. However, the foregoing
description should not be deemed to be a limitation on the scope of
the invention. Accordingly, various modifications, adaptations, and
alternatives may occur to one skilled in the art without departing
from the spirit and scope of the claimed inventive concept.
All of the patents, articles, and texts mentioned above are
incorporated herein by reference.
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